Nickel positive electrode active material and nickel metal hydride storage battery

ABSTRACT

A nickel metal hydride storage battery having an excellent charging efficiency at high temperatures is provided by using a positive electrode comprising nickel hydroxide particles and at least one rare earth compound obtainable by treating a rare earth oxide with an aqueous alkaline solution and an oxidizing agent.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a positive electrode activematerial that can be used in an alkaline storage battery and a nickelmetal hydride storage battery.

[0003] 2. Description of Related Art

[0004] With the recent spread of portable devices, alkaline storagebatteries are demanded to have higher capacity. Particularly, nickelmetal hydride storage batteries are secondary batteries, which comprisepositive electrodes mainly composed of nickel hydroxide and negativeelectrodes mainly composed of a hydrogen-absorbing alloy, have spread assecondary batteries of high capacity and high reliability.

[0005] Conventional positive electrodes for alkaline storage batterieswill be explained below.

[0006] The positive electrodes for alkaline storage batteries areroughly classified into two types that are sintered and unsintered. Theformer type of positive electrode is prepared by sintering a corematerial such as a punching metal and a nickel powder to obtain a nickelsintered substrate having a porosity of about 80%, impregnating theresulting substrate with an aqueous solution of a nickel salt such asaqueous nickel nitrate solution and then dipping the substrate in anaqueous alkaline solution, thereby to produce nickel hydroxide in theporous nickel sintered substrate. The positive electrode thus producedhas a limited substrate porosity, which makes it difficult to increasethe porosity. It is thus impossible to increase the content of theactive material to fill in. This difficulty was a limit of conventionalstorage batteries in improving their capacity.

[0007] The latter positive electrodes are those disclosed in, e.g.,JP-A-50-36935, which are obtained by filling nickel hydroxide as anactive material in a sponge-like three-dimensionally continuous nickelmetal-made porous substrate having a porosity of 95% or more. This typehas now been widely used for secondary batteries as positive electrodesof high capacity.

[0008] For the unsintered positive electrodes of the latter type, it wasproposed to fill spherical nickel hydroxide particles in a poroussubstrate, in terms of the demanded higher capacity. More specifically,the unsintered positive electrodes are obtained by filling sphericalnickel hydroxide particles having a particle diameter of several toseveral tens μm in the porous part (a pore size of approximately 200 to500 μm) of the sponge-like porous substrate.

[0009] Nickel hydroxide particles, which are located around the skeletonof the nickel metal, maintain a conductive network so thatcharging/discharging response proceeds smoothly. However, the responseof nickel hydroxide particles, which are apart from the skeleton, is notsatisfactorily smooth.

[0010] In order to improve a utilization ratio of the filled nickelhydroxide in the unsintered positive electrodes, a conductive agent isemployed in addition to nickel hydroxide as an active material, wherebythe spherical nickel hydroxide particles are electrically connected witheach other.

[0011] Cobalt compounds such as cobalt hydroxide and cobalt monoxide,metallic cobalt, metallic nickel and the like are used as the conductiveagent. Thus, it becomes possible to fill the active material at a highdensity in unsintered type positive electrodes, and the capacity can beincreased as compared with the sintered type positive electrodes.

[0012] Furthermore, JP-A 8-148145 discloses a method for producing anactive material of a positive electrode for high capacity nickel metalhydride storage batteries that are excellent in overdischargecharacteristics and meet the market demand for improvement of cyclecharacteristics, which comprises coating a cobalt compound on an activematerial nickel hydroxide and subjecting the cobalt compound to analkali oxidation treatment to convert the compound to a higher ordercobalt oxide. JP-A 9-73900 discloses an improvement of the above method.

[0013] According to these methods, the nickel hydroxide powders coatedwith the cobalt compound are sprayed with an aqueous alkaline solutionunder fluidization or dispersion in the heated air. As a result, it hasbecome possible to make alkaline storage batteries of high energydensity, which are improved in utilization ratio of an active materialand battery characteristics such as high rate discharge characteristicsas compared with the conventional methods in which the cobalt compoundis added as an external additive.

[0014] Moreover, in alkaline storage batteries, a phenomenon ofreduction of charging efficiency occurs when a temperature of thebatteries is high. For solving this problem, the electrolyte used innickel metal hydride storage batteries is optimized. Further, calciumcompounds or rare earth oxides such as yttrium oxide and ytterbium oxidethat improve the high-temperature charging efficiency are added topositive electrode active materials. This is disclosed, for example, inJP-A-9-92279.

BRIEF SUMMARY OF THE INVENTION

[0015] However, even though such conventional additives are added topositive electrodes in an increased amount to enhance the capacity andimprove the high-temperature charging efficiency, it is difficult toimprove the charging efficiency any further.

[0016] Therefore, in light of the foregoing problems, the main object ofthe present invention is to provide a nickel metal hydride storagebattery having an improved charging efficiency at high temperatures evenin a smaller amount of an additive that will be attained throughactivation of the additive.

[0017] To achieve the foregoing object, the present invention provides anickel metal hydride storage battery that uses a positive electrodecomprising an active material containing nickel hydroxide particles andat least one rare earth compound obtainable by treating a rare earthoxide with an aqueous alkaline solution and an oxidizing agent.

[0018] Other objects and advantages of the present invention shallbecome more apparent from the following description of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0019]FIG. 1 is a graph which shows a relation between the temperatureof the battery and the utilization ratio in Example 1.

[0020]FIG. 2 is a graph which shows a relation between the content ofpowders and the utilization ratio in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention relates to a nickel positive electrodeactive material comprising nickel hydroxide particles and at least onerare earth compound obtainable by treating a rare earth oxide with anaqueous alkaline solution and an oxidizing agent.

[0022] The rare earth oxide is activated by treating with an aqueousalkaline solution and an oxidizing agent. By using this activated rareearth compound, the discharging efficiency at high temperatures can beimproved even in a smaller amount of the additive.

[0023] The rare earth oxide is typically represented by formula M₂O₃,wherein M is a rare earth element. The rare earth oxide include theoxides of scandium, yttrium, promethium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium and lutetium, preferablythe oxides of yttrium, lutetium, ytterbium, holmium, erbium and,thulium, more preferably, the oxides of yttrium, lutetium and ytterbium.These oxides may also be used in an appropriate combination.

[0024] The aqueous alkaline solution is preferably an aqueous solutioncontaining at least one member selected from lithium hydroxide, sodiumhydroxide and potassium hydroxide.

[0025] Preferably, the oxidizing agent contains at least one of anaqueous sodium hypochlorite solution and an aqueous potassiumhypochlorite solution.

[0026] The rare earth compound is added preferably in a total amount of0.1 to 4.0% by weight based on the nickel hydroxide particles.

[0027] When two or more members of the rare earth compound are employed,for instance, when yttrium and lutetium compounds are used incombination, the amount of each compound preferably meets 50≧X≧5, whenthe weights of yttrium and lutetium compounds are (100-X) % by weightand X % by weight, respectively.

[0028] Further when, e.g., ytterbium and lutetium compounds are used,the amount of each compound preferably meets 50≧X≧5, when the weights ofytterbium and lutetium compounds are (100-X) % by weight and X % byweight, respectively.

[0029] The present invention further relates to a nickel metal hydridestorage battery comprising a positive electrode containing nickelhydroxide particles and the additives described above, a negativeelectrode mainly composed of a hydrogen-absorbing alloy and a separator.The nickel hydroxide particles, hydrogen-absorbing alloys and separatorshave no particular limitation to their constituents and hence, anymaterials known in the art may be used. As the nickel hydroxideparticles, there may be used, e.g., nickel hydroxide solid solutionparticles in which metal ions such as cobalt, zinc or cadmium ions orthe like are dissolved to form a solid solution and if necessary anddesired, a cobalt compound such as cobalt hydroxide or cobalt monoxide,metallic cobalt, metallic nickel, etc. may also added as a conductiveagent.

[0030] While the present invention is not limited to any theory ormechanism, the inventors presume as follows.

[0031] According to JP-A 9-92279, rare earth oxides are used asadditives to positive electrodes. The rare earth oxides added thatenters into a battery are converted into the hydroxides but some aredissolved in an electrolyte though it is in a trace amount. In thisreaction, H₂O in the electrolyte is consumed. The charging efficiency ofthe battery is dependent on a concentration of the electrolyte. When theelectrolyte concentration becomes higher, the charging efficiencydecreases. The consumption of H₂O in the electrolyte inside the batteryresults in increasing the electrolyte concentration in the battery,which might reduce the charging efficiency. Therefore, according to thepresent invention, the rare earth oxide is previously treated outsidethe battery.

[0032] In addition, by treating the rare earth oxide with the aqueousalkaline solution and the oxidizing agent outside the battery, it isassumed that the rare earth oxide would form a rare earth hydroxideprecursor having a higher activity. Rare earth hydroxides are highlycrystalline, whereas the rare earth hydroxide precursors used in thepresent invention have disordered crystalline structures compared withthose of the rare earth hydroxides. The rare earth hydroxide precursorsare considered to be coordinated with an alkali or water molecule. It isthus likely that such precursors would possess a larger number of activesites at the interface with the electrolyte compared with the rare earthhydroxides.

[0033] Therefore, according to the present invention, a rare earthhydroxide precursor, preferably, an yttrium hydroxide precursor, alutetium hydroxide precursor or an ytterbium hydroxide precursor isemployed as an additive. In the specification, the term “rare earthhydroxide precursor” is used to mean a rare earth compound that isobtained by treating a rare earth oxide with the aqueous alkalinesolution and the oxidizing agent. The precursor may also containunreacted rare earth oxides or rare earth hydroxides in such an amountthat does not impair the objects of the invention.

[0034] It is assumed that the rare hydroxide precursors would bedistinguishable from the rare earth hydroxide and the rare earth oxide,for example, based on the change of the weights. The oxide does notchange its weight so much by heating up to about 400° C., but thehydroxide changes its weight around 200° C. to 300° C., since thehydroxide alters to the oxide at the temperatures. The precursor wouldshow the weight change around 100° C., because of the elimination of thephysical absorbed water at about 100° C. and the elimination of thecrystallization water over 100° C.

[0035] Having thus generally described the present invention, thefollowing specific examples are provided to illustrate the invention.The examples are not intended to limit the scope of the invention in anyway.

EXAMPLES Example 1

[0036] After 5 g of yttrium oxide was added to 200 cm³ of 30 wt %aqueous sodium hydroxide solution, the mixture was stirred. In theresulting suspension was gradually added 100 cm³ of 20% aqueous sodiumhypochlorite solution. After bubbling of oxygen was completed, thesolution was filtered and the precipitate was washed with water. Theprecipitate was dried with a vacuum drier to give yttrium hydroxideprecursor.

[0037] Next, 300 g of nickel hydroxide powders, 30 g of cobalt hydroxidepowders, 6 g of zinc oxide and 3 g of the powders obtained by the aboveprocedure were mixed to prepare a paste. A foamed metal was filled withthe paste, followed by drying and rolling to make a positive electrodeplate. After rolling, the positive electrode plate had a thickness ofabout 750 μm. A theoretical capacity of the electrode was 1300 mAh (thetheoretical capacity was calculated based on nickel hydroxide as having289 mAh/g assuming that nickel hydroxide causes one electron reaction).

[0038] Then, a paste for a negative electrode was prepared by mixing ahydrogen-absorbing alloy of AB₅ type with 1 wt % of a carbon material, 1wt % of PTFE and water. The paste was coated on a foamed metal, followedby drying and rolling. A thickness of the negative electrode thusprepared was 420 μm after rolling. The electrode had a theoreticalcapacity of 1900 mAh.

[0039] As a separator, nonwoven fabric made of polypropylene was used. Athickness of the separator was 130 μm.

[0040] These positive and negative electrodes and the separatordescribed above were disposed in the order of positiveelectrode-separator-negative electrode-separator. The entire system wasrolled in a spiral form and encased in a battery case of A4 size. Thecase was filled with a given volume of an alkali electrolyte solution.Thereafter, the case was sealed at the upper part with a sealing plateto make a sealed type nickel metal hydride storage battery.

[0041] The battery was charged at 130 nA in the atmosphere of 25° C. for15 hours and then discharged at 260 mA until discharge voltage reached 1V. A utilization ratio (the ratio of actual dischargecapacity/theoretical capacity of positive electrode, which is shown inpercentage) determined from the discharge capacity under the givenconditions was 98%. This battery is referred to as battery A of thepresent invention in Example 1.

[0042] For comparison, two types of batteries were prepared.

[0043] One battery for comparison was prepared in a manner similar toExample 1 except that yttrium oxide was used without any treatment, inplace of the yttrium hydroxide precursor obtained in Example 1 bytreating yttrium oxide with the aqueous alkaline solution and theoxidizing agent. This battery is referred to as battery X.

[0044] Another battery for comparison was prepared using a positiveelectrode to which no yttrium oxide was added. This battery is referredto as battery Y. The batteries X and Y showed a utilization rate of 98%,respectively, in the atmosphere of 25° C.

[0045] Next, these batteries were charged at 130 mA in the atmospheresof 25° C., 45° C., 50° C., 55° C. and 60° C., respectively. Thetemperature was then lowered to 25° C. and the batteries were dischargedat 260 mA.

[0046]FIG. 1 shows a utilization ratio at each temperature, in whichsolid line, chain line and dotted line designate utilization ratios ofbattery A and batteries X and Y for comparison, respectively.

[0047] As is clear from FIG. 1, in the battery of the present invention,the charging efficiency increased more at elevated temperatures, ascompared to the batteries to which known yttrium oxide was added.

Example 2

[0048] Positive electrodes to which the yttrium hydroxide precursorpowders prepared as in Example 1 were added in the amounts of 0.1, 0.2,0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 wt % were made and nickel metalhydride storage batteries of A4 size were fabricated as in Example 1.

[0049] These batteries were charged at 130 mA in an atmosphere of 55°C., and then the temperature was lowered to 25° C. The batteries werethen discharged at 260 mA. The utilization ratio in this case is shownin FIG. 2. As is clear from FIG. 2, the optimum range for the amount ofthe precursor added to improve the charging efficiency is noted and apreferred range is found to be 0.1 to 4.0 wt %.

Example 3

[0050] Five grams of yttrium oxide and 5 g of lutetium oxide were addedto 300 cm³ of 30 wt % aqueous sodium hydroxide solution followed bystirring. In the resulting suspension was gradually added 200 cm³ of 20%aqueous sodium hypochlorite solution. After bubbling of oxygen wascompleted, the solution was filtered and the precipitate was washed withwater. The precipitate was treated in the same manner as in Example 1 toprepare a battery added with 2 wt % of the precursor thus obtained. Autilization ratio of the battery at 55° C. was 92%. A similar effect wasobtained even when the precursor was mixed in 50:50.

[0051] In the Examples above, the powders obtained by treating yttriumoxide and lutetium oxide with the aqueous alkaline solution and theoxidizing agent were used but similar effects can be obtained also withytterbium oxide.

[0052] In the Examples described above, sodium hydroxide was used as theaqueous alkaline solution but for this purpose, lithium hydroxide andpotassium hydroxide can be used singly or by mixing them.

[0053] In the Examples described above, sodium hypochlorite was used asthe oxidizing agent but similar effects can also be obtained whenpotassium hypochlorite is employed.

[0054] The effects described above are considered to be obtained bytreating yttrium oxide, ytterbium oxide and/or lutetium oxide with theaqueous alkaline solution and the oxidizing agent. Even when the otherimpurities, e.g., rare earth oxides, transition metal oxides, alkalineearth elements, etc. are contained, these impurities do not adverselyaffect the effects of the invention.

[0055] The addition of cobalt oxide or zinc oxide used in the Examplesabove should not be deemed to limit the invention but is merely given byway of examples.

[0056] As described above, the nickel metal hydride storage batteryusing the positive electrode of the invention which is added with therare earth compound treated with the aqueous alkaline solution and theoxidizing agent provides markedly improved efficiencies especially athigh temperatures, resulting in an immensely valuable industrialutility.

What is claimed is:
 1. A nickel positive electrode active materialcomprising nickel hydroxide particles and at least one rare earthcompound obtainable by treating a rare earth oxide with an aqueousalkaline solution and an oxidizing agent.
 2. A nickel positive electrodeactive material according to claim 1 , wherein the rare earth compoundis at least one selected from the group consisting of yttrium compoundobtainable by treating yttrium oxide with an aqueous alkaline solutionand an oxidizing agent, a lutetium compound obtainable by treatinglutetium oxide with an aqueous alkaline solution and an oxidizing agent,and a ytterbium compound obtainable by treating ytterbium oxide with anaqueous alkaline solution and an oxidizing agent.
 3. A nickel positiveelectrode active material according to claim 1 , wherein a total amountof the rare earth compound is in the range of 0.1 to 4.0 wt % based onthe nickel hydroxide particles.
 4. A nickel positive electrode activematerial according to claim 2 , wherein the rare earth compound is acombination of the yttrium compound and the lutetium compound, whereinthe two compounds meet 50≧X≧5, when weights of the yttrium compound andthe lutetium compound are (100-X) % by weight and X % by weight,respectively.
 5. A nickel positive electrode active material accordingto claim 2 , wherein the rare earth compound is a combination of theytterbium compound and the lutetium compound, wherein the two compoundsmeet 50≧X≧5, when weights of the ytterbium compound and the lutetiumcompound are (100-X) % by weight and X % by weight, respectively.
 6. Anickel positive electrode active material according to claim 1 , whereinthe aqueous alkaline solution is an aqueous solution containing at leastone selected from the group consisting of lithium hydroxide, sodiumhydroxide and potassium hydroxide.
 7. A nickel positive electrode activematerial according to claim 1 , wherein the oxidizing agent contains atleast one selected from the group consisting of an aqueous sodiumhypochlorite solution and an aqueous potassium hypochlorite solution. 8.A nickel metal hydride storage battery comprising a positive electrodemainly composed of a positive electrode active material of claim 1 , anegative electrode mainly composed of a hydrogen-absorbing alloy and aseparator.